Method and system for forming a halftone screen

ABSTRACT

A method of forming a halftone screen comprising representing each of a first, second, third and fourth tone range of increasing darkness by forming a plurality of dots arranged in a grid; the dots representing the second tone range larger than the dots representing the first tone range; the dots representing the third tone range having a body section and at least one extension extending toward a or respective nearest neighbor(s), the extension(s) narrower in width than the body section, the dots representing the third tone range substantially the same size as the dots representing the second tone range; and the dots representing the fourth tone range having a body section and at least one extension extending toward a or respective nearest neighbor(s), the extension(s) greater in width than the extension(s) associated with the dots representing the third tone range. A halftone screen and a printing system are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application pursuant to 35 U.S.C.§371 of International Application No. PCT/NZ2012/000225, filed Dec. 3,2012, which claims priority to U.S. Provisional Patent Application Ser.No. 61/566,284, filed Dec. 2, 2011, the disclosures of which are herebyincorporated by reference herein.

FIELD OF INVENTION

The invention relates to methods and systems for forming a halftonescreen.

BACKGROUND TO THE INVENTION

Colour images are typically printed using three or more colours andblack. These colours are known as process colours. Common processcolours comprise cyan, magenta and yellow. Black is also used to improvethe printing of dark areas. A large range or gamut of colours can bereproduced by printing the process colours in different combinations,together with black. Some printing techniques use other colours eitherin addition or in substitution for cyan, magenta yellow and black.

A given colour is reproduced by adjusting the amount of each of theprocess colours and black. This is achieved by adjusting the ratio ofink to white paper for each process colour and black. Generally theratio of ink to white paper is controlled by printing each processcolour in a series of dots that vary in either size or frequency. Thepattern of dots for each process colour is called a halftone pattern, ahalftone screen or a screen.

The combination of the individual halftone screens for each processcolour and black results in the appearance of the full colour image.

In traditional amplitude modulated screening, the ratio of ink to whitepaper is controlled by varying the size of individual dots. The dots arearranged on a grid that is defined by the number of lines per inch. Thisis known as the screen ruling or screen frequency. The grid can beeither orthogonal or non-orthogonal.

The grids for each process colour and black will generally be ondifferent angles known as screen angles. In irrational screening, whichis a commonly used technique, three orthogonal grids of the same screenruling are rotated to intervals of 30° relative to each other. The threegrids typically represent cyan, magenta and black respectively. A fourthscreen for the yellow colour may be of a slightly different screenruling or frequency which is generally arranged at 15° to two of theother screens. This arrangement results in a pattern that incorporates aseries of circular features known as rosettes.

An individual grid comprises an array of cells known as dot cells. Eachdot cell generally contains a maximum of one dot. In the case of anorthogonal grid, the dot cells are square.

There are a variety of dot shapes that are used to create halftonescreens. Each dot shape is made up of n individual shapes that beginwith a white dot cell representing 0% tone. The dot shape is formed andincreases in size as the tone darkens (the level of density increases).Eventually a dot cell is formed that is completely filled with inkrepresenting the 100% tone, i.e., the dot shape covers 100% of the areaof the dot cell. As the area covered by the dot increases, the densityof ink is said to increase or, alternatively, the tone is said to becomedarker. Often n will be 255 since this number of individual dot shapes,also representing the number of density levels, is an industry norm.

Three traditionally used dot shapes are round dots, elliptical dots andsquare dots. More commonly, the square dot is positioned so that itssides are oriented at 45 degrees to the edges of the dot cell containingthe dot. Square dots on this orientation make up a chequerboard patternnear the mid-tone point (50% density) when all four corners touch. Thisorientation can be referred to as the 45 degree orientation.

An alternative use of the square dot is where the dot is positioned sothat its sides are orientated parallel to the respective adjacent edgesof the dot cell containing the dot. Square dots on this orientationresult in a grid of non-image white lines in the mid to three-quartertones. This orientation can be referred to as the parallel orientation.

The square dot on the parallel orientation has the potential to producea sharper high contrast look especially in the mid tones. A disadvantageof the square dot on the parallel orientation is that, as the dotapproaches higher density levels, the non printed area becomes verythin. It is difficult to accurately print individual dots with such finenon printed areas between them. The result is that ink can bridge thenarrow gap producing a jump in tone, thereby reducing the effectivenumber of density levels available. This in turn limits the gamut ofcolours that can reliably be reproduced. This means that the square doton the parallel orientation is effectively rendered impractical for manyprinting processes.

It is an object of preferred embodiments of the present invention toaddress some of the aforementioned disadvantages of the square dot onthe parallel orientation. An additional and/or alternative object is toat least provide the public with a useful choice.

SUMMARY OF INVENTION

The invention in one embodiment comprises a method of forming a halftonescreen the method comprising representing a first tone range by forminga plurality of dots arranged in a grid;

representing a second tone range by forming a plurality of dots arrangedin a grid, the dots representing the second tone range larger in sizethan the dots representing the first tone range, the second tone rangedarker than the first tone range; representing a third tone range byforming a plurality of dots arranged in a grid, the dots having a bodysection and at least one extension extending toward a or respectivenearest neighbour(s), the extension(s) narrower in width than the bodysection of the dots, the body sections of the dots representing thethird tone range substantially the same size as the larger dots withinthe second tone range, the third tone range darker than the second tonerange; and representing a fourth tone range by forming a plurality ofdots arranged in a grid, the dots having a body section and at least oneextension extending toward a or respective nearest neighbour(s), theextension(s) greater in width than the extension(s) associated with thedots representing the third tone range, the fourth tone range darkerthan the third tone range.

The term ‘comprising’ as used in this specification and claims means‘consisting at least in part of’. When interpreting statements in thisspecification and claims which include the term ‘comprising’, otherfeatures besides the features prefaced by this term in each statementcan also be present. Related terms such as ‘comprise’ and ‘comprised’are to be interpreted in similar manner.

Preferably the dots representing the first tone range are substantiallysquare on the parallel orientation. Preferably the substantially squaredots representing the first tone range have at least partly concavesides.

Preferably the dots representing the first tone range are substantiallycircular.

Preferably the dots representing the first tone range are substantiallycross-shaped.

Preferably the dots representing the second tone range are substantiallysquare on the parallel orientation. Preferably the substantially squaredots representing the second tone range have at least partly concavesides.

Preferably the body sections of the dots representing the third tonerange are substantially square on the parallel orientation. Preferablythe substantially square body sections of the dots representing thethird tone range have at least partly concave sides.

Preferably at least one extension extending from the substantiallysquare body sections of the dots representing the third tone rangeextend(s) from a or respective midpoint(s) of respective linesconnecting two adjacent corners of the substantially square bodysections.

Preferably the at least one extension extending from the body sectionsof the dots representing the third tone range extend(s) from an edge ofthe dot at a point that is in line with the centre of the dot and thecentre of a or respective nearest neighbour(s).

Preferably the width of the extensions extending from the body sectionsof the dots representing the third tone range at any point is inverselyproportional to the distance of the point from the body sections.

Preferably the width of the extensions extending from the body sectionsof the dots representing the third tone range is substantially uniformalong the length of the extension.

Preferably the width of extensions extending from the body sections ofthe dot representing the third tone range is non-uniform along thelength of the extension.

Preferably the method further comprises representing an intermediatetone range by forming a plurality of dots arranged in a grid, theintermediate tone range darker than the second tone range, the thirdtone range darker than the intermediate tone range, the dots having abody section and one or more portions positioned outwardly of andseparate from the body section.

Preferably the method further comprises forming a cross-shapednon-printed area between at least some of the dots representing thethird tone range.

Preferably the method further comprises forming cross-shaped non-printedareas between at least some of the dots representing the fourth tonerange, the cross-shaped non-printed areas between the dots within thefourth tone range smaller in area than the cross-shaped non-printedareas between the dots within the third tone range.

Preferably the body sections of the dots representing the fourth tonerange are substantially square on the parallel orientation. Preferablythe substantially square dots representing the fourth tone range have atleast partly concave sides.

Preferably at least one extension from the substantially square bodysections of the dots representing the fourth tone range extend(s) from aor respective midpoint(s) of respective lines connecting two adjacentcorners of the substantially square body sections.

Preferably the at least one extension extending from the body sectionsof the dots representing the fourth tone range extend(s) from an edge ofthe dot at a point that is in line with the centre of the dot and thecentre of a or respective nearest neighbour(s).

Preferably the dots representing the third tone range are connected toat least one nearest neighbour with the respective extensions.

Preferably the dots representing the fourth tone range are connected toat least one nearest neighbour with the respective extensions.

Preferably the method further comprises representing a fifth tone rangeby forming a plurality of non-printed areas smaller in area thannon-printed areas in the first to fourth tone ranges.

In another embodiment the invention comprises a tangible computerreadable medium having stored thereon computer-executable instructionsfor performing a method of forming a halftone screen, the methodcomprising representing a first tone range by forming a plurality ofdots arranged in a grid; representing a second tone range by forming aplurality of dots arranged in a grid, the dots representing the secondtone range larger in size than the dots representing the first tonerange, the second tone range darker than the first tone range;representing a third tone range by forming a plurality of dots arrangedin a grid, the dots having a body section and at least one extensionextending toward a or respective nearest neighbour(s), the extension(s)narrower in width than the body section of the dots, the body sectionsof the dots representing the third tone range substantially the samesize as the larger dots within the second tone range, the third tonerange darker than the second tone range; and representing a fourth tonerange by forming a plurality of dots arranged in a grid, the dots havinga body section and at least one extension extending toward a orrespective nearest neighbour(s), the extension(s) greater in width thanthe extension(s) associated with the dots representing the third tonerange, the fourth tone range darker than the third tone range.

Preferably the dots representing the first tone range are substantiallysquare on the parallel orientation. Preferably the substantially squaredots representing the first tone range have at least partly concavesides.

Preferably the dots representing the first tone range are substantiallycircular.

Preferably the dots representing the first tone range are substantiallycross-shaped.

Preferably the dots representing the second tone range are substantiallysquare on the parallel orientation. Preferably the substantially squaredots representing the second tone range have at least partly concavesides.

Preferably the body sections of the dots representing the third tonerange are substantially square on the parallel orientation. Preferablythe substantially square body sections of the dots representing thethird tone range have at least partly concave sides.

Preferably at least one extension extending from the substantiallysquare body sections of the dots representing the third tone rangeextend(s) from a or respective midpoint(s) of respective linesconnecting two adjacent corners of the substantially square bodysections.

Preferably the at least one extension extending from the body sectionsof the dots representing the third tone range extend(s) from an edge ofthe dot at a point that is in line with the centre of the dot and thecentre of a or respective nearest neighbour(s).

Preferably the width of the extensions extending from the body sectionsof the dots representing the third tone range at any point is inverselyproportional to the distance of the point from the body sections.

Preferably the width of the extensions extending from the body sectionsof the dots representing the third tone range is substantially uniformalong the length of the extension.

Preferably the width of extensions extending from the body sections ofthe dot representing the third tone range is non-uniform along thelength of the extension.

Preferably the method further comprises representing an intermediatetone range by forming a plurality of dots arranged in a grid, theintermediate tone range darker than the second tone range, the thirdtone range darker than the intermediate tone range, the dots having abody section and one or more portions positioned outwardly of andseparate from the body section.

Preferably the method further comprises forming a cross-shapednon-printed area between at least some of the dots representing thethird tone range.

Preferably the method further comprises forming cross-shaped non-printedareas between at least some of the dots representing the fourth tonerange, the cross-shaped non-printed areas between the dots within thefourth tone range smaller in area than the cross-shaped non-printedareas between the dots within the third tone range.

Preferably the body sections of the dots representing the fourth tonerange are substantially square on the parallel orientation. Preferablythe substantially square dots representing the fourth tone range have atleast partly concave sides.

Preferably at least one extension from the substantially square bodysections of the dots representing the fourth tone range extend(s) from aor respective midpoint(s) of respective lines connecting two adjacentcorners of the substantially square body sections.

Preferably the at least one extension extending from the body sectionsof the dots representing the fourth tone range extend(s) from an edge ofthe dot at a point that is in line with the centre of the dot and thecentre of a or respective nearest neighbour(s).

Preferably the dots representing the third tone range are connected toeach nearest neighbour with the respective extensions.

Preferably the dots representing the fourth tone range are connected toeach nearest neighbour with the respective extensions.

Preferably the method further comprises representing a fifth tone rangeby forming a plurality of non-printed areas smaller in area thannon-printed areas in the first to fourth tone ranges.

In a further embodiment the invention comprises a halftone screencomprising a first tone range comprising a plurality of dots arranged ina grid; a second tone range comprising a plurality of dots arranged in agrid, the dots representing the second tone range larger in size thanthe dots representing the first tone range, the second tone range darkerthan the first tone range; a third tone range comprising a plurality ofdots arranged in a grid, the dots having a body section and at least oneextension extending toward a or respective nearest neighbour(s), theextension(s) narrower in width than the body section of the dots, thebody sections of the dots representing the third tone rangesubstantially the same size as the larger dots within the second tonerange, the third tone range darker than the second tone range; and afourth tone range comprising a plurality of dots arranged in a grid, thedots having a body section and at least one extension extending toward aor respective nearest neighbour(s), the extension(s) greater in widththan the extension(s) associated with the dots representing the thirdtone range, the fourth tone range darker than the third tone range.

In another embodiment the invention comprises a printing systemcomprising a memory; a processor programmed to form at least onehalftone screen and store the halftone screen in the memory, theprocessor further programmed to: represent a first tone range by forminga plurality of dots arranged in a grid; represent a second tone range byforming a plurality of dots arranged in a grid, the dots representingthe second tone range larger in size than the dots representing thefirst tone range, the second tone range darker than the first tonerange; represent a third tone range by forming a plurality of dotsarranged in a grid, the dots having a body section and at least oneextension extending toward a or respective nearest neighbour(s), theextension(s) narrower in width than the body section of the dots, thebody sections of the dots representing the third tone rangesubstantially the same size as the larger dots within the second tonerange, the third tone range darker than the second tone range; andrepresent a fourth tone range by forming a plurality of dots arranged ina grid, the dots having a body section and at least one extensionextending toward a or respective nearest neighbour(s), the extension(s)greater in width than the extension(s) associated with the dotsrepresenting the third tone range, the fourth tone range darker than thethird tone range.

BRIEF DESCRIPTION OF FIGURES

Preferred forms of a method and system for forming a halftone screenwill now be described with reference to the accompanying figures inwhich:

FIG. 1 shows conventional equipment used in preparing halftone screens.

FIG. 2 shows a method for preparing a halftone screen.

FIG. 3 shows a conversion process from the methods shown in FIG. 2.

FIG. 4 shows a gradation illustrating the range of dot shapes of theinvention.

FIG. 5 shows preferred form dot shapes in a first tone range from FIG.4.

FIG. 6 shows an alternative preferred form dot shape in a first tonerange from FIG. 4.

FIG. 7 shows a preferred form dot forming part of a second tone rangefrom FIG. 4.

FIG. 8 shows a preferred form dot forming part of a third tone rangefrom FIG. 4.

FIG. 8A shows another preferred form dot forming part of the third tonerange from FIG. 4.

FIG. 8B shows another preferred form dot forming part of the third tonerange from FIG. 4.

FIG. 9 shows a preferred form dot forming part of a fourth tone rangefrom FIG. 4.

FIGS. 9A, 9B and 9C show other preferred form dots within the fourthtone range from FIG. 4.

FIG. 10 shows a preferred form dot forming part of a fifth tone rangefrom FIG. 4.

FIG. 11 shows a preferred form intermediate tone range shape precedingthe dot shape of FIG. 8A.

DETAILED DESCRIPTION

Referring to the drawings it will be appreciated that the various novelpatterns and techniques must here be printed in black and white andmagnified significantly for the purposes of a clear description. It willalso be appreciated that the patterns and various shapes of the printedand non-printed areas may be implemented by an appropriate mathematicalconstruction of a spot function or a threshold matrix using a wide rangeof hardware and software that is already available. A schematicindication of hardware and software systems that are suitable forimplementing the invention will be given. Details are left to theskilled reader.

FIG. 1 shows a schematic collection of computer-based or otherwisecomputer controlled equipment that might be used in production ofhalftone screens and in producing a result in printed form. A wide rangeof equipment and software is currently in use or under development andthese items are shown by way of example only and for the purposes ofexplanation.

Part of the processing that relates most closely to the inventionusually takes place in a computer processor arrangement 100. This may bea separate item of equipment as shown, or may be part of a more generalitem such as a processing device shown as a desktop computer 110.

Computer processing arrangement 100 includes further components such asa main processor 120 and data storage capacity 130. The device mayoptionally include a dedicated processor 140 for high speed processingof image data.

Halftone patterns or screens 150 produced by the processor are indicatedas physical forms in the CMYK system. This physical form might be filmfrom an imagesetter or laser plotter 160 for example, or printing platesfor a press 170.

Alternatively the halftone screens may be represented by computer datain a portable memory storage component such as solid state, optical ormagnetic storage media. The halftone screens may exist as data files invarious formats which can then be saved or output to other devices suchas platesetters, or digital printers.

An original image may be input to the equipment in various ways such asfrom a photograph or other tangible artwork scanned by a digital scanner180 or an image captured in digital form by a digital camera 190.Alternatively the original image is created initially on a computersystem such as desktop computer 110 interfaced to printer 195.

FIG. 2 outlines a process for generating and printing halftone screens.An original digital image is created in step 200 on a computer device.The image data may be created using a graphics program and stored in aformat which is native to the device such as a particular vector orbitmap format.

Parts of the image may also be imported from other sources such asclipart, a digital camera, or scanned photographs when composing adesktop publication for example. In step 205, the native data isconverted into a standard format such as POSTSCRIPT or PDF which can beinterpreted by an output device such as a local laser printer with aparticular resolution.

The standard format is converted into pixel data in step 210 where theimage is represented by an array of digital values. This process isknown as rasteriztation and usually occurs inside a raster imageprocessing program or RIP.

A wide variety of patterns and processes are then available forconversion of the pixel data to halftone screen data. Detailsappropriate to a particular image are selected automatically or by anoperator in step 215. The conversion processes take place in step 220 byway of a computer system 100 such as that indicated in FIG. 1.

The patterns are generally created from the pixel information by a RIPor other software. The program calculates the locations, spacings, sizesand shapes of the dots for each colour separation.

An operator normally has a range of standard patterns installed on theequipment and may be able to develop variations of those patterns insome cases. Traditional calculations involve screen frequencies andangles although more sophisticated equipment enables patterns to becreated without reference to these parameters. The operator is able toproof and modify the selection of patterns to minimise various effectssuch as moire, colour shift and tone jump according to the subsequentreproduction technique.

The RIP or other software then stores the halftone screen data in aformat such as black and white bitmap file in computer memory.

Copies of the original image may then be printed in step 225 or sent toa platesetter, stored in an electronic format such as a TIFF file, ortransferred elsewhere from the stored data as required, depending on themanner in which the image will be published and the equipment which isavailable.

FIG. 3 outlines the conversion process of step 220 from FIG. 2. The stepis generally carried out by known RIP or other software. The processfirst receives pixel data including colour and tone information in step300. Each pixel includes data which enables determination of a tonevalue for each of the colours in a chosen colour system. This coloursystem is usually CMYK. The tone value is determined for a small arealocated at the point represented by the pixel in the coloured image.

The generation of a halftone screen for each of the colours produces alarge amount of data that may be stored in a common computer file or inseparate files. This data usually takes the form of binary on/offinstructions in relation to the output grid of a printing device.

In steps 305, 310, 315 and 320 the pixel data is parsed for each of therequired colours to determine halftone screen information for thatcolour. Mathematical constructs called spot functions or thresholdmatrices are used in step 310 to determine the halftone patterns thathave been selected for the colours.

Each pixel is compared with a threshold matrix for the particularcolour. Binary data relating to the output grid of the particularprinting device is determined and stored in an appropriate computer filein step 315. Eventually the colour separation is complete and thehalftone screen data can be stored or output to a device such as aplatesetter or a printer in step 325 when required.

The techniques described below provide a sequence of dot shapes thatseek to retain and improve upon the advantages of the square shaped doton a parallel orientation in the mid tone. As described above, theadvantage of the square shaped dot on the parallel orientation is thepotential to produce a sharper higher contrast look in the mid tones.The techniques described below seek to at least address the traditionaldisadvantage of such square dot shapes of the non printed area beingthin and difficult to accurately print at high tone levels.

The square shaped dot on the parallel orientation improves the mid toneby partially suppressing the appearance of the rosette. This occursbecause the square shaped dot on the parallel orientation puts ink intothe centre of the rosette. Other commonly used dot shapes such as roundor elliptical dots tend to leave white space at the centre of therosette. The contrast between the white space at the centre and the inkaround the edge makes rosettes more noticeable in print.

The techniques described below seek to retain the penetration of inkinto the corners of the dot cell for as much of the range of the dotshape as possible, while allowing for a non printed area that suits thelimitations of the printing process.

FIG. 4 shows a gradation that shows a series of ranges of dot shapesrepresenting darkening tone from top to bottom. The halftone screen istypically formed by representing a first tone range 400 by forming aplurality of dots arranged in a grid. It is anticipated that the grid iseither orthogonal or non-orthogonal. In one embodiment the grid issubstantially regular. In a further embodiment the grid is at leastpartly distorted.

Subsequent tone ranges are also shown in the gradation for example asecond tone range 405, a third tone range 410 and a fourth tone range415. There is an optional fifth tone range 420. These five tone rangesare described further below with reference to FIG. 4 and subsequentfigures.

It is anticipated that the dots in the first tone range 400 comprisevarious shapes. In one embodiment most of the dots are substantiallysquare. In another embodiment most of the dots are substantially roundor circular.

The dot shapes shown in FIG. 4 and subsequent figures relate to a singlecolour. In practice there may be four different screens, one for each ofcyan, magenta, black and yellow and each on a different screen angle. Itis anticipated that the dot shapes shown below are used for at least oneof these colours. In FIG. 4 the dots are shown on an orthogonal grid ata screen angle of 45 degrees.

FIG. 5 shows preferred form dots shown in the first tone range 400. Oneexample dot is shown at 500 within a dot cell 505. The dot cell 505represents the maximum size that a dot 500 can assume. It is anticipatedthat there is generally one dot 500 per dot cell 505.

Dot 500 and neighbouring dots are shown as substantially square with atleast one and preferably all four sides at least partly concave.

FIG. 6 shows an alternative preferred form dot 600 represented within adot cell 605. Dot 600 forms part of first tone range 400. As shown inFIG. 6 the dot 600 and neighbouring dots are substantially circular.

It will be appreciated that other dot shapes are possible within thefirst tone range 400. One example is where some or all of the dots aresubstantially cross-shaped. It is anticipated that the four arms of thecross are substantially the same length.

Referring to FIG. 4, the method further includes representing a secondtone range 405 by forming a plurality of dots arranged in a grid. Thedots representing the second tone range are typically larger in sizethan the dots representing the first tone range 400. As is apparent fromFIG. 4, the second tone range 405 is darker than the first tone range400.

FIG. 7 shows a magnified view of a dot 700 within a dot cell 705. Dot700 is representative of a dot forming part of the second tone range405. As is apparent in FIG. 7, the dot 700 is larger in size than dot500 or dot 600 and takes up more of the dot cell 705. The preferred formshape of the dot 700 is substantially square. As shown, it is preferredthat one or preferably all of the sides of dot 700 are at least partlyconcave.

Referring to FIG. 4, the halftone screen includes a third tone range 410which is represented by forming a plurality of dots arranged in a grid.The dots have a body section and at least one extension extending towarda/or respective nearest neighbour(s).

FIG. 8 shows a more detailed view of a dot represented within the thirdtone range 410. As shown in FIG. 8, the dot 800 is presented within thedot cell 805. Dot 800 has a body section 810. Dot 800 includes anextension 815 that extends toward its nearest neighbour. As shown inFIG. 8 there are preferably four extensions shown at 815, 815A, 815B and815C. Where there is more than one extension 815, each extension 815extends toward its nearest neighbour.

As shown in FIG. 8, each extension 815 extends to the edge of the dotcell 805 so that it touches corresponding extensions extending towarddot 800 from nearest neighbours. However, in practice, some dots in thethird tone range will have shorter extensions that do not reach the edgeof the dot cell. The extensions grow to reach the edge of the dot celltowards the end of the tone range.

Preferably the extensions 815 are narrower in width than the bodysection 810 of the dot 800. As will be apparent from the figures, thebody section 810 of dot 800 is substantially the same size as the largerof the dots 700 within the second tone range. As shown in FIG. 4, thethird tone range 410 is darker than the second tone range 405.

The body section 810 of dot 800 is preferably a substantially squareshape. As shown in FIG. 8, the body section 810 preferably includes atleast one and preferably four concave sides.

Extension 815 for example preferably extends from body section 810 fromthe midpoint of a line connecting corner 820 and corner 830 of the bodysection 810. As shown in FIG. 8, preferably each of the extensions 815extend from the midpoint of the side of the body 810 from which theyextend.

The extensions 815 are preferably tapered. The width of the extensions815 at any point is inversely proportional to the distance of the pointfrom the body section 810.

As also shown in FIG. 8, there is a cross-shaped non printed area 840between dot 800 and three of its neighbours. Preferably there are fourcross-shaped non printed areas 840, 840A, 840B and 840C between dot 800and its neighbours.

FIG. 8A shows a detailed view of an alternative example of a dotrepresented within the third tone range 410. As shown in FIG. 8A the dot800A is presented within dot cell 805A. Dot 800A has a body section810A. Dot 800A includes at least one and preferably four extensions in asimilar way to that shown in FIG. 8. At least one extension 815A extendstoward its nearest neighbour.

As can be seen in FIG. 8A, the width of one or more of the extensions815A extending from the body section 810A is/are non-uniform along thelength of the extension(s).

As shown in FIG. 8A, the extensions 815A extend to the edge of the dotcell 805A so that the extensions 815A touch respective correspondingextensions extending toward dot 800A from nearest neighbours.

It is anticipated that at least some dots of lighter tone than dot 800Aare associated with at least one portion that is/are not connected todot 800A. Alternatively or additionally at least some dots of lightertone than dot 800A include at least one extension that does not touchrespective corresponding extensions extending toward dot 800A fromnearest neighbours.

Preferably the extensions 815A are narrower in width than the bodysection 810A of the dot 800A. As will be apparent from the figures, thebody section 810A of dot 800A is substantially the same size as thelarger of the dots 700 within the second tone range.

The body section 810A of dot 800A is preferably a substantially squareshape. As shown in FIG. 8A, the body section 810A preferably includes atleast one and preferably four concave sides. Extension(s) 815Apreferably extend from the body section 810A from the midpoint of a sideof the body 810A from which they extend.

The extensions 815A are preferably tapered. The width of the extensions815A at any point is proportional to the distance of the point from thebody section 810A. The width of the extensions 815A is greater towardsthe extremities of the extensions.

FIG. 8B shows a detailed view of a further alternative example of a dotrepresented within the third tone range 410. As shown in FIG. 8B the dot800B is presented within dot cell 805B. Dot 800B has a body section.

The dot 800B includes at least one and preferably four extensions in asimilar way to that shown in FIG. 8. At least one extension extendstoward its nearest neighbour.

As can be seen in FIG. 8B, the width of one or more of the extensions815B extending from the body section of dot 800B is/are non-uniformalong the length of the extension(s). The extension(s) 815 arepreferably tapered. The width of the extension(s) at any point isproportional to the distance of the point from the body section 800B.The width of the extension(s) is/are narrower towards the extremities ofthe extensions.

As shown in FIG. 4, there is a fourth tone range 415. The fourth tonerange 415 is represented by forming a plurality of dots arranged in agrid. As shown in FIG. 9 a dot 900 of tone range 415 is shown presentedwithin a dot cell 905. Dot 900 has a body section 910 and at least oneextension 915 extending from body section 910 toward its nearestneighbour. Preferably there are further extensions 915A, 915B and 915Cthat extend from the body 910 to respective nearest neighbours.

Extension 915 preferably extends from body section 910 from the midpointof a line connecting corner 920 and 930 of the body section 910. Asshown in FIG. 9, preferably each of the extensions 915 extend from themidpoint of the side of the body section 910 from which they extend.

Extensions 915 are greater in width than extensions 815 from FIG. 8. Thebody section 910 is approximately the same size as body section 810 andthe larger of the dots 700 within the second tone range. However theincreased thickness of the extensions results in smaller adjacent crossshapes or non printed areas 940, 940A, 940B and 940C. This in turnresults in the fourth tone range 415 being darker than the third tonerange 410.

As shown in FIG. 9, the body section 910 is substantially square andpreferably has at least one and preferably four partly concave sides.

It is anticipated that the extensions 815 and 915 in some embodimentswill extend to the extreme edge of the dot cell within which the dot isrepresented. This will have the effect of connecting the extensions tonearest neighbours.

FIGS. 9A, 9B and 9C show alternative examples of preferred form dotsincluded within the fourth tone range. The dots in these figures wouldtypically be used where alternative dot 800B shown in FIG. 8B is alsoused.

Referring to FIG. 4 and FIG. 10, preferably there is a fifth tone range420 in which a dot 1000 shown presented within a dot cell 1005 includesa body section 1010. The cross-shaped non printed areas 1015, 1015A,1015B and 1015C are each smaller in area than the non-printed areas inany of the first to fourth tone ranges 400, 405, 410 and 415. Thiseffect is caused by a widening of the extensions.

FIG. 11 shows a preferred form intermediate tone range. Thisintermediate tone range preferably precedes the dot shape shown in FIG.8A. As shown in FIG. 11, the dot 1100 is presented within the dot cell1105. Dot 1100 has a body section 1110. Dot 1100 has at least oneportion 1115 that is positioned outwardly of and separate from the bodysection 1110. As shown in FIG. 11 there are preferably four portionsshown at 1115, 1115A, 1115B and 1115C. Where there is more than oneportion 1115, each portion 1115 is positioned toward a nearestneighbour.

The portions 1115 shown in FIG. 11 are generally semi-circular and arepositioned adjacent the border of the dot cell 1105 so that the portionstouch corresponding portions associated with nearest neighbours.However, in practice, some portions in the intermediate tone range willtouch body section 1110 and/or will not touch corresponding portionsassociated with nearest neighbours. It is anticipated that portions 1115could be any shape and are not required to be semi-circular.

Preferably the portions 1115 are narrower in width than the body section1110. The body section 1110 of dot 1100 is substantially the same sizeas the larger of the dots 700 within the second tone range. Theintermediate tone range is darker than the second tone range.Furthermore, the third tone range is darker than the intermediate tonerange.

The body section 1110 of dot 1100 is preferably a substantially squareshape. As shown in FIG. 11, the body section 1110 preferably includes atleast one and preferably four concave sides.

The portions 1115 are positioned adjacent to a midpoint of a lineconnecting corner 1120 and corner 1130 of the body section 1110. Asshown in FIG. 11, preferably each of the portions 1115, 1115A, 1115B and1115C are positioned adjacent to the midpoint of the side of the body1110 closest to the portion.

The foregoing describes the invention including preferred forms thereof.Modifications and improvements as would be obvious to those skilled inthe art are intended to be incorporated in the scope hereof, as definedby the accompanying claims.

The invention claimed is:
 1. A method of forming a halftone screen, themethod comprising: representing a first tone range by forming aplurality of dots arranged in a grid; representing a second tone rangeby forming a plurality of dots arranged in a grid, the dots representingthe second tone range larger in size than the dots representing thefirst tone range, the second tone range darker than the first tonerange; representing a third tone range by forming a plurality of dotsarranged in a grid having cross-shaped non-printed areas between atleast some of the dots representing the third tone range, the dotshaving a body section and at least one extension extending toward arespective nearest neighbour or respective nearest neighbours, theextension(s) narrower in width than the body sections of the dots, thebody sections of the dots representing the third tone rangesubstantially the same size as the larger dots within the second tonerange, the third tone range darker than the second tone range;representing a fourth tone range by forming a plurality of dots arrangedin a grid, the dots having a body section and at least one extensionextending toward a respective nearest neighbour or respective nearestneighbours, the extension(s) greater in width than the extension(s)associated with the dots representing the third tone range, the fourthtone range darker than the third tone range; and using a printing systemto print an image using at least one of the first tone range, the secondtone range, the third tone range or the fourth tone range.
 2. The methodof claim 1 further comprising forming cross-shaped non-printed areasbetween at least some of the dots representing the fourth tone range,the cross-shaped non-printed areas between the dots representing thefourth tone range smaller in area than the cross-shaped non-printedareas between the dots representing the third tone range.
 3. The methodof claim 1 wherein the body sections of the dots representing the fourthtone range are substantially square on a parallel orientation.
 4. Themethod of claim 3 wherein the substantially square dots representing thefourth tone range have at least partly concave sides.
 5. The method ofclaim 3 wherein at least one extension extending from the substantiallysquare body sections of the dots representing the fourth tone rangeextend(s) from a respective midpoint or respective midpoints ofrespective lines connecting two adjacent corners of the substantiallysquare body sections.
 6. The method of claim 1 wherein the at least oneextension extending from the body sections of the dots representing thefourth tone range extend(s) from an edge of a dot at a point that is inline with a centre of the dot and a centre of a respective nearestneighbour or respective nearest neighbours.
 7. The method of claim 1wherein the dots representing the third tone range are connected to atleast one nearest neighbour with the respective extensions.
 8. Themethod of claim 1 wherein the dots representing the fourth tone rangeare connected to at least one nearest neighbour with the respectiveextensions.
 9. The method of claim 1 further comprising representing afifth tone range by forming a plurality of non-printed areas smaller inarea than the non-printed areas in the third and fourth tone ranges. 10.The method of claim 1 wherein the dots representing the first tone rangeare substantially square on a parallel orientation.
 11. The method ofclaim 10 wherein the substantially square dots representing the firsttone range have at least partly concave sides.
 12. The method of claim 1wherein the dots representing the first tone range are substantiallycircular.
 13. The method of claim 1 wherein the dots representing thefirst tone range are substantially cross-shaped.
 14. The method of claim1 wherein the dots representing the second tone range are substantiallysquare on a parallel orientation.
 15. The method of claim 14 wherein thesubstantially square dots representing the second tone range have atleast partly concave sides.
 16. The method of claim 1 wherein the bodysections of the dots representing the third tone range are substantiallysquare on a parallel orientation.
 17. The method of claim 16 wherein thesubstantially square body sections of the dots representing the thirdtone range have at least partly concave sides.
 18. The method of claim16 wherein at least one extension extending from the substantiallysquare body sections of the dots representing the third tone rangeextend(s) from a respective midpoint or respective midpoints ofrespective lines connecting two adjacent corners of the substantiallysquare body sections.
 19. The method of claim 1 wherein the at least oneextension extending from the body sections of the dots representing thethird tone range extend(s) from an edge of a dot at a point that is inline with a centre of the dot and a centre of a respective nearestneighbour or respective nearest neighbours.
 20. The method of claim 1wherein the width of the extensions extending from the body sections ofthe dots representing the third tone range at any point is inverselyproportional to a distance of the point from the body sections.
 21. Themethod of claim 1 wherein the width of the extensions extending from thebody sections of the dots representing the third tone range issubstantially uniform along a length of the extension.
 22. The method ofclaim 1 wherein the width of extensions extending from the body sectionsof the dots representing the third tone range is non-uniform along alength of the extension.
 23. The method of claim 1 further comprising:representing an intermediate tone range by forming a plurality of dotsarranged in a grid, the intermediate tone range darker than the secondtone range, the third tone range darker than the intermediate tonerange, the dots having a body section and one or more portionspositioned outwardly of and separate from the body section.
 24. Atangible non-transitory computer readable medium having stored thereoncomputer-executable instructions for performing a method of forming ahalftone screen, the method comprising: representing a first tone rangeby forming a plurality of dots arranged in a grid; representing a secondtone range by forming a plurality of dots arranged in a grid, the dotsrepresenting the second tone range larger in size than the dotsrepresenting the first tone range, the second tone range darker than thefirst tone range; representing a third tone range by forming a pluralityof dots arranged in a grid having cross-shaped non-printed areas betweenat least some of the dots representing the third tone range, the dotshaving a body section and at least one extension extending toward arespective nearest neighbour or respective nearest neighbours, theextension(s) narrower in width than the body sections of the dots, thebody sections of the dots representing the third tone rangesubstantially the same size as the larger dots within the second tonerange, the third tone range darker than the second tone range; andrepresenting a fourth tone range by forming a plurality of dots arrangedin a grid, the dots having a body section and at least one extensionextending toward a respective nearest neighbour or respective nearestneighbours, the extension(s) greater in width than the extension(s)associated with the dots representing the third tone range, the fourthtone range darker than the third tone range.
 25. The medium of claim 24wherein the method further comprises forming cross-shaped non-printedareas between at least some of the dots representing the fourth tonerange, the cross-shaped non-printed areas between the dots representingthe fourth tone range smaller in area than the cross-shaped non-printedareas between the dots representing the third tone range.
 26. The mediumof claim 24 wherein the body sections of the dots representing thefourth tone range are substantially square on a parallel orientation.27. The medium of claim 26 wherein the substantially square dotsrepresenting the fourth tone range have at least partly concave sides.28. The medium of claim 26 wherein at least one extension from thesubstantially square body sections of the dots representing the fourthtone range extend(s) from a respective midpoint or respective midpointsof respective lines connecting two adjacent corners of the substantiallysquare body sections.
 29. The medium of claim 24 wherein the at leastone extension extending from the body sections of the dots representingthe fourth tone range extend(s) from an edge of a dot at a point that isin line with a centre of the dot and a centre of a respective nearestneighbour or respective nearest neighbours.
 30. The medium of claim 24wherein the dots representing the third tone range are connected to eachnearest neighbour with the respective extensions.
 31. The medium ofclaim 24 wherein the dots representing the fourth tone range areconnected to each nearest neighbour with the respective extensions. 32.The medium of claim 24, wherein the method further comprisesrepresenting a fifth tone range by forming a plurality of non-printedareas smaller in area than the non-printed areas in the third and fourthtone ranges.
 33. The medium of claim 24 wherein the dots representingthe first tone range are substantially square on a parallel orientation.34. The medium of claim 33 wherein the substantially square dotsrepresenting the first tone range have at least partly concave sides.35. The medium of claim 24 wherein the dots representing the first tonerange are substantially circular.
 36. The medium of claim 24 wherein thedots representing the first tone range are substantially cross-shaped.37. The medium of claim 24 wherein the dots representing the second tonerange are substantially square on a parallel orientation.
 38. The mediumof claim 37 wherein the substantially square dots representing thesecond tone range have at least partly concave sides.
 39. The medium ofclaim 24 wherein the body sections of the dots representing the thirdtone range are substantially square on a parallel orientation.
 40. Themedium of claim 39 wherein the substantially square body sections of thedots representing the third tone range have at least partly concavesides.
 41. The medium of claim 39 wherein at least one extensionextending from the substantially square body sections of the dotsrepresenting the third tone range extend(s) from a respective midpointor respective midpoints of respective lines connecting two adjacentcorners of the substantially square body sections.
 42. The medium ofclaim 24 wherein the at least one extension extending from the bodysections of the dots representing the third tone range extend(s) from anedge of a dot at a point that is in line with a centre of the dot and acentre of a respective nearest neighbour or respective nearestneighbours.
 43. The medium of claim 24 wherein the width of theextensions extending from the body sections of the dots representing thethird tone range at any point is inversely proportional to a distance ofthe point from the body sections.
 44. The medium of claim 24 wherein thewidth of the extensions extending from the body sections of the dotsrepresenting the third tone range is substantially uniform along alength of the extension.
 45. The medium of claim 24 wherein the width ofextensions extending from the body sections of the dots representing thethird tone range is non-uniform along a length of the extension.
 46. Themedium of claim 24 wherein the method further comprises: representing anintermediate tone range by forming a plurality of dots arranged in agrid, the intermediate tone range darker than the second tone range, thethird tone range darker than the intermediate tone range, the dotshaving a body section and one or more portions positioned outwardly ofand separate from the body section.
 47. A printing system for generatinga halftone screen, the halftone screen comprising: a first tone rangecomprising a plurality of dots arranged in a grid; a second tone rangecomprising a plurality of dots arranged in a grid, the dots representingthe second tone range larger in size than the dots representing thefirst tone range, the second tone range darker than the first tonerange; a third tone range comprising a plurality of dots arranged in agrid having cross-shaped non-printed areas between at least some of thedots representing the third tone range, the dots having a body sectionand at least one extension extending toward a respective nearestneighbour or respective nearest neighbours, the extension(s) narrower inwidth than the body sections of the dots, the body sections of the dotsrepresenting the third tone range substantially the same size as thelarger dots representing the second tone range, the third tone rangedarker than the second tone range; and a fourth tone range comprising aplurality of dots arranged in a grid, the dots having a body section andat least one extension extending toward a respective nearest neighbouror respective nearest neighbours, the extension(s) greater in width thanthe extension(s) associated with the dots representing the third tonerange, the fourth tone range darker than the third tone range.
 48. Aprinting system comprising: a memory; a processor programmed to form atleast one halftone screen and store the halftone screen in the memory,the processor further programmed to: represent a first tone range byforming a plurality of dots arranged in a grid; represent a second tonerange by forming a plurality of dots arranged in a grid, the dotsrepresenting the second tone range larger in size than the dotsrepresenting the first tone range, the second tone range darker than thefirst tone range; represent a third tone range by forming a plurality ofdots arranged in a grid having cross-shaped non-printed areas between atleast some of the dots representing the third tone range, the dotshaving a body section and at least one extension extending toward arespective nearest neighbour or respective nearest neighbours, theextension(s) narrower in width than the body sections of the dots, thebody sections of the dots representing the third tone rangesubstantially the same size as the larger dots representing the secondtone range, the third tone range darker than the second tone range; andrepresent a fourth tone range by forming a plurality of dots arranged ina grid, the dots having a body section and at least one extensionextending toward a respective nearest neighbour or respective nearestneighbours, the extension(s) greater in width than the extension(s)associated with the dots representing the third tone range, the fourthtone range darker than the third tone range.